The present invention relates to an organic electroluminescence element and a manufacturing method thereof. More specifically, the present invention relates to an organic electroluminescence element which is used, for example, in a display or display element for cellular phones or in various light sources and which is an electroluminescence element driven over a wide brightness range from low brightness to high brightness in usage for a light source or the like.
The organic electroluminescence element is a light-emitting device utilizing an electroluminescence phenomenon of a solid fluorescent substance and is partially put into practical use as a small display.
The organic electroluminescence element can be classified into several groups by the material used for the light-emitting layer. One representative example is a low-molecular organic electroluminescence element using an organic compound with a low molecular weight for the light-emitting layer, which is manufactured mainly using vacuum vapor deposition. Another example is a polymer organic electroluminescence element using a polymer compound for the light-emitting layer.
In the case of a polymer organic electroluminescence element, use of a solution prepared by dissolving materials constituting each functional layer enables film formation by a wet coating method such as spin coating, inkjet coating, nozzle coating, cap coating, spraying and printing. Thanks to this simple process, the wet coating method is attracting attention as a technique that can be expected to realize cost reduction and large screen area.
A typical polymer organic electroluminescence element is fabricated by stacking a plurality of functional layers such as charge injection layer and light-emitting layer. The construction and fabrication procedure of a representative polymer organic electroluminescence element are described below.
For example, as shown in FIG. 8, on a glass substrate 100 where an ITO (indium tin oxide) film is formed as the anode 1122, a PEDOT:PSS (a mixture of polythiophene and polystyrenesulfonic acid; hereinafter referred to as PEDT) thin film is formed as the electron (hole) injection layer 1123 by a spin coating method or the like. PEDT is a material that is a de-facto standard as the charge injection layer; and functions as the hole injection layer when disposed on the anode side.
On the PEDT layer, an interlayer 1124 composed of an organic polymer material is provided. The interlayer is, for example, a copolymer of a triphenylamine derivative and polyfluorene. For example, poly-(2,7(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-sec-butylphenyl)imino)-1,4-phenylene)) indicated by TFB is used. This compound is excellent in the hole injection property and at the same time, has an electron blocking function. Therefore use of the compound brings about elevation of the light emission efficiency and improvement of the driving lifetime. Next, polyphenylene vinylene (hereinafter, indicated by PPV) and a derivative thereof, or polyfluorene and a derivative thereof, is film-formed as the light-emitting layer 1125 by a spin coating method or the like. On the light-emitting layer, injection of an organic material into the lowest unoccupied molecular orbital (LUMO) is efficiently performed using an electron injecting material 1126 of a material with a small work function such as an alkali metal or alkaline earth metal (e.g., Ba, Ca, Mg, Li, Cs) or a fluoride, oxide or carbonate of the metal above, e.g., LiF, BaO, CsCO3. Thereafter, a metal electrode 1127 such as Al or Ag is provided as the cathode. The film of these metals is formed by a vacuum vapor deposition method, a sputtering method or a wet coating method:
In this way, the polymer organic electroluminescence element can be fabricated by a simple and easy process and because of this excellent property, its application to various uses is expected. However, PEDT:PSS used as the hole injection layer is an acidic water-soluble chemical compound, and this incurs the following problems: first, a problem in view of apparatus, that is, the compound corrodes metal portions of the coating apparatus used; secondly, a problem ascribable to bad wettability to a partition wall formed mainly of a resist material and provided to divide a picture element; and thirdly, a serious problem that because the compound has bad wettability to a material which has a light-emitting function and is dissolved in an organic solvent, when the picture element is divided into fine pixels of a display or the like, the uniformity of the coated film within a pixel becomes insufficient to impair the light emission uniformity or allow easy occurrence of short circuiting. Also, it is known that chemical deterioration is caused by the injection of an electric charge and adversely affects the lifetime.
Reduction of the light emission intensity, that is, deterioration, of the polymer organic electroluminescence element proceeds in proportion to the product of the electric energization time and the current flowing in the element, but its details are not yet elucidated and intensive studies thereon are being made.
Reduction of the light emission intensity is presumed to be brought about by various causes, but the cause is considered to be a combination of various factors such as stability of the light-emitting material itself or a functional layer (e.g., hole injection layer, electron injection layer) against an electron or a hole, side reaction from an exciton, thermal stability, stability of interface between layers, diffusion of a material due to heat, and oxidation of a cathode material.
In the polymer electroluminescence element, as described above, deterioration of PEDT is considered to be one of main causes of the reduction of the light emission intensity. As previously indicated, PEDT is a mixture of two polymer substances, that is, polystyrenesulfonic acid and polythiophene. The former is ionic and the latter has local polarity in the polymer chain. These two polymer substances are loosely bound through a Coulomb interaction ascribable to the electric charge anisotropy, and excellent charge injection characteristics are thereby exerted.
In order for PEDT to exert excellent characteristics, an intimate interaction between those two substances is indispensable, but in general, a mixture of polymer substances is likely to cause phase separation due to a subtle difference of solubility in a solvent. This is no exception to PEDT. Occurrence of phase separation indicates relatively easy breaking of the loose binding between two polymers. The phase separation suggests that when driven in an organic electroluminescence element PEDT may be unstable. Also, as a result of phase separation, a component not contributing to the binding, in particular, an ionic component, may diffuse due to an electric field associated with electric energization and adversely affect other functional layers. In this way, despite excellent charge injection characteristics, PEDT is not a stable substance by any means.
Against the above-described concern about PEDT, the present inventors have made various experiments and, based on the experiment results, proposed to form a transition metal oxide such as molybdenum oxide MoO3 between an anode and a light-emitting layer instead of PEDT, and good injection characteristics can be thereby obtained (see, JP-2005-203340).
The problem relevant to the hole injection layer is greatly improved by the above invention, but from the standpoint of light emission efficiency, more improvements are being demanded, because the light emission efficiency sometimes decreases depending on the material used.
There have been also proposed a light-emitting diode having a laminated film of MoS2 and MoO3 formed by a coating method and a light-emitting element containing an electrode having a structure of ITO/MoS2/MoO3/polymer organic semiconductor layer with MoS2 being annealed (see, Journal of Applied Physics, Vol. 92, 7556-7563 (2002) and Advanced Materials 2002, Vol. 14, 265-268). In both of these, MoS2 is formed by a coating method and therefore, there is a problem that not only formation with a uniform thickness is difficult due to surface bulging in the pattern edge but also MoS2 allows for a large leakage current to increase the leakage current with an adjacent pixel and is hard to integrally form particularly when achieving microfabrication and high integration.
Also, it is reported that when tungsten oxide is evaporated on ITO by an electron beam method and heat-treated at 450° C. to vapor-deposit a low molecular-type organic EL material, the light emission efficiency is enhanced (see, Synthetic metals, 151, 141-146 (2005)). But this technique cannot be used because the annealing temperature is as high as 450° C. and the high temperature adversely affects other constituent members such as insulating film or partition wall for the separation of a picture element in fabricating a display or the like. Furthermore, because the optimal film thickness is as very thin as 1.5 nm and the film thickness dependency is also large, there is a serious drawback of variation when a large-size substrate of second or greater generation is used.
A case of using nickel oxide is also known (see, Thin Solid Films, 515, 5099-5102 (2007)). This is a method of vapor-depositing a 10 nm-thick Ni metal and then heat-treating it at 500° C. to effect conversion into nickel oxide. The publication above indicates that emission efficiency is enhanced by performing a heat treatment and the optimal condition is 4 hours. However, also in the technique of this publication, the annealing temperature is high and, since metallic Ni is underlying, there is a problem that cross-talk occurs if the underlayer is entirely oxidized. In addition, it is not indicated that high efficiency can be achieved compared with the conventionally employed hole injection layer such as starburst amine or copper phthalocyanine.
In the case of using a large-size substrate, the large film thickness dependency greatly affects the yield and leads to incapability of stable mass production.
In this way, in the structure above, a hole injection layer composed of a transition metal oxide having a film thickness of approximately from 10 to 100 nm is used on an anode, and a functional layer such as light-emitting layer is formed thereon. The functional layer is mainly formed from an interlayer and a light-emitting layer or an electron transport layer and since the interlayer used here is a thin film having a thickness of around about 20 nm and contains almost the same organic solvent as the light-emitting material, intermixing between layers often occurs. Furthermore, the interlayer is required to have an electron blocking function so as to cause an electron injected from a cathode to stay in the light-emitting layer but cannot completely block an electron due to an intermixing problem between layers or a problem in view of chemical structure and a part of electrons are allowed to pass into an anode without being used for recombination, as a result, there arises a problem such as failure in obtaining sufficient emission efficiency.
Under these circumstances, the present invention has been made and an object of the present invention is to improve the light emission characteristics of a device when a transition metal oxide is used for the hole injection layer.
In particular, an object of the present invention is to enhance the electron blocking characteristics of a transition metal oxide.